1. Field of the Invention
This invention relates to planar diode and solar cell products. More particularly, it relates to such products made from low-cost refined metallurgical silicon.
2. Description of the Prior Art
The development of new techniques and products for the low-cost utilization of non-polluting sources of energy is of paramount national and world-wide interest. Solar energy is among the energy sources of greatest interest because of its non-polluting nature and of its abundant, non-diminishing availability. Two separate approaches have been utilized in efforts to develop solar energy as a suitable energy source for satisfying significant portions of the ever-increasing energy requirements of modern, industrial societies. In one approach, solar energy is converted into thermal energy, while the second approach involves the conversion of solar energy into electricity by means of the photovoltaic effect upon the absorption of sunlight by so-called solar cells. The present invention relates to the second approach and to the development of low-cost silicon materials for use in such solar cells.
Silicon solar cells, the most commonly employed devices based on said photovoltaic effect, have been employed reliably in space craft applications for many years. For such applications and for industrial and commercial applications in general, crystals of high purity, semiconductor grade silicon are commonly utilized. Such high purity, high perfection silicon is prepared by rather costly procedures involving converting metallurgical grade silicon to trichlorosilane, which is then reduced to produce polycrystalline, semiconductor grade silicon from which single crystals can be grown. The costs associated with the production of such high purity, high perfection crystals are high. For example, polycrystalline semiconductor grade silicon made from metallurgical grade silicon costing about $0.50/lb. will presently cost on the order of about $30/lb. and up. A single crystal is grown from this semiconductor grade material, the ends of the single crystal ingot or boule are cut off, and the boule is sawed, etched and polished to produce polished wafers for solar cell application, with mechanical breakage and electronic imperfection reducing the amount of useable material obtained. As a result of such processing, less than 20% of the original polycrystalline, semiconductor grade silicon will generally be recovered in the form of useable wafers of single crystal material. The overall cost of such useable material is, accordingly, presently on the order of about $300/lb. and up. Because of the relatively large area requirements involved in solar cell applications, such material costs are a significant factor in the overall economics of such applications.
The economic feasibility of utilizing solar cell technology for significant portions of the present and prospective needs for replenishable, non-polluting energy sources would be enhanced, therefore, if the utilization of high cost, high purity, high perfection single crystal wafers could be avoided. Previous efforts to refine metallurgical silicon for other applications, however, have not resulted in the production of materials that can be utilized in solar cells although the electronic characteristics of various grades of silicons for solar cells are less stringent than, for example, such silicons as employed for complex circuitry in the electronics industry.
Metallurgical silicon has heretofore been refined by slag oxidation to obtain a grade of metallurgical silicon or ferrosilicon advantageous as an alloying additive in the manufacturing of steels. As indicated in the Barber et al patent, U.S. Pat. No. 2,797,988, and elsewhere, the slag oxidation approach has been utilized to remove impurities and thus to purify and refine silicon having an iron content therein such that the refined product constitutes a ferrosilicon in which iron is considered an integral part of the final refined product. For use of silicon as a substrate in planar diodes and solar cells made therefrom, however, it is necessary that the refined silicon have as low an iron content as possible, as such iron is a deleterious impurity in a solar cell material. As indicated above, this circumstance is in contradistinction to the benign nature of the iron content of the refined silicon product as utilized in the steel industry.
Silicon has also been purified heretofore by the dissolution and subsequent precipitation of silicon from a liquid metal system. Such purification, taught by Litz, U.S. Pat. No. 3,097,068 and Wakefield, U.S. Pat. No. 3,933,981, involve the retention of silicon impurities by the liquid metal solvent when dissolution takes place at a higher temperature and, subsequently, the temperature is lowered to precipitate a relatively pure silicon. Such a silicon product is not suitable for use as a substrate in solar cell applications, however, since the liquid metal of the solvent phase is present as an impurity in the product silicon so obtained in this processing technique.
While Litz states that the product of such refining by liquid metal solvent is too impure for use in transistors, use for rectifiers and solar batteries is indicated. The product, however, is said to contain 200 to 700 ppm of aluminum, with no mention of its iron content. Such a level of aluminum would render the product purified by the Litz technique unsuitable for solar cell application. Litz also discloses a further lengthy and expensive procedure using silicon tetrachloride to reduce the aluminum content to 9 ppm.
Other efforts to develop acceptable solar cell materials have likewise resulted either in high cost, or high impurity levels such that acceptable efficiencies can not be obtained, or a combination of these disadvantages. An acceptable efficiency for a low-cost, relatively impure silicon would, of course, very likely represent some loss from the high purity single crystal material made from semiconductor grade silicon. Such high-cost material is capable of providing efficiencies of about 13-14%. The lower cost of relatively impure silicon material, particularly multigrained material, might well constitute an advantageous trade-off enhancing the overall technical-economic feasibility of utilizing silicon solar cell technology in significant commercial operations.
The practical lower limit for economic solar cell efficiencies is about 7-8%. One attempt to produce an acceptable low-cost material has involved the pulling of ribbons from a melt of metallurgical grade silicon by known techniques. While such ribbons pulled from semiconductor grade material have obtained efficiencies of up to 10%, the ribbon pulled from metallurgical silicon is relatively dirty and impure, with solar cell efficiencies obtained from such ribbon being limited to about 5%.
Another approach has involved the pulling of boules, in a multiple series of refining steps, from metallurgical grade silicon as a starting material by the known Czochralski-pulling technique. By re-melting and re-pulling refined, multigrained silicon in a several stage process, a single crystal refined silicon is ultimately obtained that is capable of achieving efficiencies of about 8%. The cost of such material, however, is relatively high because of the multiple pulling steps involved.
Chu, U.S. Pat. No. 3,961,997, discloses the fabrication of low-cost solar cell substrates from metallurgical grade, polycrystalline silicon. In this approach, successive layers of polycrystalline silicon containing appropriate dopants are deposited over substrates of metallurgical grade silicon, graphite or steel coated with particular diffusion barrier materials. The resulting products contain a high level of impurities such that efficiencies obtainable by modifications of this approach have not exceeded about 3-5%.
A genuine need thus exists for low-cost, relatively impure silicon products suitable for use in solar cells of acceptable efficiency. The resulting low-cost solar cells should preferably have efficiencies in excess of about 10%, with 7-8% representing a practical lower limit of efficiency as indicated above.
It is an object of this invention, therefore, to provide low-cost refined metallurgical silicon products suitable for solar cell applications.
It is another object of the invention to provide refined metallurgical silicon of a relatively impure nature as compared with semiconductor grade material while having useful properties for said solar cell applications.
It is a further object of the invention to provide multigrained refined metallurgical silicon useful for solar cell applications or readily convertible to such useful silicon material.
With these and other objects in mind, the invention is hereinafter described, the novel features thereof being particularly pointed out in the appended claims.